Carrier wave modulation



June 12, 1945. w. R. BENNETT CARRIER WAVE MODULATION Filed Feb. 26, 1944SIDEBAND 0U TPU T 1 T 2 CARRIER SUPPL Y CON TA l/V/NG 2 ND I-IARMON/C/9,. m Imw m 1 40 MM B RE-SUL TANT CARR/ER FUNDAMENTAL -2 ND HARMOiV/Q l'1 L-Ram TANT CARR/ER FIG. 3

I FUNDAMENTAL 2 ND HARHON/C //v l ENTOR W R. BENNETT A TTOR/VEV ferentfrequencies.

Patented June 12, 1945 2,371,858 caanrna WAVE MODULATION William R.Bennett, Summit, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationFebruary 26, 1944, Serial No. 524,087

1 Claim.

The present invention relates to the modulation of carrier waves by asignal or other relatively low frequency waves and relates especially tothe reduction of signal leak or direct transmission of signal currentsthrough the modulator into the side-band output circuit or load circuit.

Signal leak becomes a particular problem in systems in which the signaland side-band frequencies coincide, since filtering can be used in caseswhere the signal and side-bands have dif- One instance that may be citedas an example in which filtering cannot be used to suppress signal leakwould be a speech frequency inverting circuit in which the invertedside-band frequencies are to coincide with or overlap the speech band.

The commonest cause of signal leak is, of course, circuit unbalance andeffort is always made to secure as perfect a balance as possible bychoosing the modulator elements to match as closely as possible andmaking the transformers as symmetrical as possible. After all possibleprecautions have been taken in balancing the clrcuit, there remains apossible source of signal leak in the presence of second harmonics inthe carrier supply which, depending upon their phase,

may have the effect of lengthening the conductivity period of themodulator in one-half cycle of the applied carrier and shortening theconductivity period in the next half cycle. This results in a netrectifying effect which is equivalent to producing some side-band outputbased on direct current, that is, some of the original signal,thusproduclng signal leak.

In accordance with the present invention this source of dissymmetry inthe operation giving rise to signal leak is eliminated by producing therequisite phase shift in the second harmonic of the carrier supplyrelative to the fundamental carrier frequency component.

The nature and objects of the invention will be more fully understoodfrom the following detailed description in connection with the attacheddrawing, in which:

Fig. 1 is a schematic circuit diagram of a double balanced ringmodulator incorporating the improvement of the present invention; and

Figs. 2 and 3 are graphs of wave forms illus-v trating the eifect ofphase relations .between the figdamental and second harmonic ofthecarrier Referring first to Fig. 1, the ring modulator containing thefour elements", A, B. C and D is connected in a double balanced circuitof known l and an output transformer II for connecting respectively, tothe signal input circuit and a sideband output circuit or load.

The windings of the transformers l0 and I I ad- Jacent the ring aresupplied with center taps between which is connected the carrier supplysource assumed to be connected to circuit I2. In

accordance with the present invention a phase shifter I3 is connected inthe circuit l2 for introducing relatively greater phase shift for thesecand harmonic than for the fundamental frequency component of thecarrier current. Ideally, the phase shifter It may be regarded asintroducing phase shift for only the second harmonic component. 7

In the usual operation of the circuit of Fig. 1, not considering for themoment the improvement feature of the present invention, the amplitudeof the carrier wave would be chosen to be large compared with themaximum amplitude of the signal wave. The effect of this is to cause thering A, B, C, D to operate as a switch which is opened and closed undercontrol of the carrier wave so as to transmit some of the signal inputwave through to the side-band output circult in respectively reversedpolarity in successive half cycles of the applied carrier wave. Forexample, when the carrier wave is in such direction as to make theimpedance of elements A and C low and the impedance of elements B and Dhigh, current flows around the circuit in the direction E, A, G, H, C,F, thus producing current flow in the primary winding of outputtransformer II in the direction indicated by the solid arrows, assumingthe signal input wave has the polarity indicated by the solid arrowsadjacent input transformer I ll. When the carrier wave reverses inpolarity, elements B and D are made to have low impedance and elements Aand C high impedance. Current then flows in the direction E, D, H, G, B,F, thus sending current through .the primary winding of the outputtransformer time in which a voltage of sufficiently high value isapplied to them from the carrier supply source. If the duration of theconductivity condition in mum! mprising an input transformer one-halfcycle of the applied carrier wave is slightly different from theduration of the conductivity in the alternate hali' cycle, a residualdirectcurrent component of the carrier is pro- 'duced by rectificationand this causes a proportionate amount of the signal to be transferredto the side-band output circuit as signal leak. Such a dissymmetrybetween alternate half cycles of the carrier waves may be caused, forexample, by the presence of second harmonic in the carrier supply.

Figs. 2 and 3 represent, with some exaggeration for purposes ofillustration, two extreme cases in which (Fig. 2) the second harmoniccomponent has such phase as to pass through zero at the same instant asthe fundamental carrier component and (Fig. 3) the second harmonic hasbeen shifted 90 degrees with respect to the case "shown in Fig. 2. Thedotted lines in each case represent the resultant carrier that isactually applied to the ring circuit, this being obtained -by directaddition of the fundamental and second harmonic components. The-brokenline rectangular curve having opposite polarity portions represents theconductivity duration of the ring modulator in the correspondingdirection. Comparison of Figs. 2 and 3 shows that the conductivityperiod is equal in the positive and negative directions in Fig. 2,whereas in Fig. 3 the conductivity in the positive direction is shorterthan that in the negative direction. The condition illustrated in Fig.2, therefore, is such as to produce no signal leak by rectification ofthe carrier wave, while the condition illustrated in Fig. 3 beach as toproduce a signal leak, since there is a net resultant girect currentcomponent produced by rectifica- The relations indicated in Fig. 3 maybe changed to those illustrated in Fig. 2 by shifting the phase of thesecond harmonic component relative to the fundamental. This may be doneby inserting the phase shifter ll of Fig. 1, which may be in the form ofan all-pass network of the lat-. tice type with inverse reactivenetworks in the series and shunt branches and with impedance" matchingthe output circuit of the oscillator. The attenuation of the networkshould be low at the carrier frequency, and the phase shift at thesecond harmonic of the carrier frequency relative to that at the carrierfrequency is preferably variable to permit the introduction of anydesired amount of phase shift diil'erence throughout a range of at least90 degrees. If it is known in any particular case that the phaserelations shown in Fig. 3 exist, then, of course, a simple 90-degreephase shifting circuit for the second harmonic component may be used atIt.

While the conditions illustrated in Figs, 2 and 3 are exaggerated, thepresence of even a small amount of second harmonic component willproduce a corresponding unsymmetrical effect in the resultant appliedcarrier wave. Where the suppression of the second harmonic cannot be orhas not been made sufliciently complete by filtering to eliminate theeffect. a phase shifter It may be used with or without a purifyingfilter or as part of a purifying filter to secure a more perfect result.

What is claimed is:

In a signal modulating circuit, a ring modulator, a source of signalcurrents and a load connected to points in said ring such as to be inconjugate relation to each other, a source of carrier waves connectedbetween mid-points in said source and load respectively, said carrierwave source delivering a relatively small amount of second harmoniccurrent with the carrier wave, and means-for controlling direct signaltransmission to said load due to inequality in conduction times of saidring in successive carrier half cycles comprising a phase shifterbetween said ring and said source of carrier waves for producing phaseshift in the second harmonic relative to the fundamental carrier'wave ofsuch amount as to cause the carrier wave fundamental and sec-- andharmonic component applied to the ring to pass through zerocoincidentally.

WIIJJAMRBENNE'I'I.

